Bone screws are regularly utilised in orthopaedic surgery for the fixation of fractured bone fragments and the like.
One common form of bone screw is a self-tapping bone screw having a screw thread that cuts into bone material forming the wall of a pre-drilled hole extending through the bone fragments to be fixed. This cutting action of the screw thread creates an internal thread in the bone, along which the bone screw thread is advanced, with engagement between the (external) screw thread and the internal bone thread fixing the screw within the bone fragments. The self-tapping bone screws typically have a series of flutes or grooves extending from adjacent the screw tip towards the screw head, creating discontinuities in the otherwise continuous bone screw thread so as to define discrete screw thread segments. The sharp leading edge of each of these thread segments, defined at the intersection between the crest of the thread segment and the trailing wall of the adjacent flute provides the cutting action of the thread.
The amount of fixation that the screw thread provides in the bone is dependent upon a number of variables. These variables include the thread surface area and volume of the interface between the external screw thread and internal bone thread, the number of full revolution thread portions engaged for each bone fragment, the friction generated between the external screw thread and internal bone thread and the quality of the bone material in which the external screw thread is engaged. The greater the number of thread portions, the greater the interface volume and the greater the friction, the greater will be the fixation and pullout strength of the bone screw. The pullout strength and fixation is, however, at times insufficient, resulting in loosening of the bone screw or backing out of the bone post-operatively.